Bias drive and noise reduction in image projectors

• US 5,471,255 A
• Filed: 11/24/1992
• Issued: 11/28/1995
• Est. Priority Date: 11/24/1992
• Status: Expired due to Term

• Current Assignee: JVC Kenwood Corporation
• Original Assignee: JVC Americas Corporation (JVC Kenwood Corporation)
• Inventors: Hagerman, James G.
• Primary Examiner(s): Groody, James J.
• Assistant Examiner(s): GRANT, CHRISTOPHER C
• Application Number:
  US07/980,681
• Time in Patent Office: 1,099 Days
• Field of Search: 358/236, 340/784, 340/784 D, 340/784 C, 340/784 F, 340/784 J, 340/784 I, 307/311, 307/296.6, 359/72, 359/84, 359/85
• US Class Current: 348/761
• CPC Class Codes: H04N 5/7441 the modulator being an array of liquid crystal cells

Abstract

Improved bias drive and noise reduction techniques for use with and image projector, such as a liquid crystal light valve image projector. A bias drive circuit produces a square wave-like signal to drive the liquid crystal light valve, for example, with a square wave signal. The square wave-like signal is phase-locked to the vertical refresh (vertical sync) signal of the projector. A frequency synthesizer is used to drive the bias drive circuit wherein the noise is interlaced by using every other field as the sync reference input to the bias drive circuit. The bias drive circuit is unique in how it reduces unwanted visual noise effects in a displayed image by waveshaping the drive signal and interlacing and phase-locking it to the vertical refresh signal of the projector. These aspects of the present invention combine to virtually eliminate visual noise in the projected image. An optional variable gain circuit may be provided that provides the ability to set the amplitude of the bias provided by the bias drive circuit to the cut-in operating point of the liquid crystal light valve.

Claims

1. A bias drive circuit for use in an image projector, said bias drive circuit comprising:

an amplifier having first and second inputs and an output, and wherein the first input of the amplifier is adapted to receive a square wave input signal;

a feedback network means coupled between the output and the second input of the amplifier for feeding back output signals provided by the bias drive circuit to the amplifier and for modifying the output signal therefrom such that its output waveform is a non-square wave that, when applied to the image projector, results in a square wave within the image projector; and

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by way of an output, said output frequency signal (f_OUT) comprising said square wave input signal received by said amplifier first input; and

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector.

2. The apparatus of claim 1 wherein the image projector comprises a liquid crystal light valve including a liquid crystal, and wherein the output waveform of the amplifier is applied across the liquid crystal of the liquid crystal light valve.

3. A bias drive circuit for use in an image projector, said bias drive circuit comprising:

an amplifier having first and second inputs and an output, and wherein the first input of the amplifier is adapted to receive a square wave input signal;

a feedback network coupled between the output and the second input of the amplifier that is adapted to feed back output signals provided by the bias drive circuit to the amplifier and modify the output signal therefrom such that its output waveform, when applied to the image projector, results in a square wave applied thereto; and

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by way of an output, said output frequency signal (f_OUT) comprising said square wave input signal received by said amplifier first input; and

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector;

wherein a divide number of the divide by N divider is an odd number and the input frequency signal is divided by two so that every other field is employed as a reference input to the bias drive circuit.

4. The apparatus of claim 3 wherein said input frequency signal comprises the vertical sync signal of the image projector.

5. The apparatus of claim 3 further comprising a variable gain circuit that is adapted to set the amplitude of the bias provided by the bias drive circuit to a cut-in operating point of the image projector.

6. A bias drive circuit for use in an image projector comprising a liquid crystal light valve including a liquid crystal, said bias drive circuit comprising:

an amplifier having first and second inputs and an output, and wherein the first input of the amplifier is adapted to receive a square wave input signal;

a feedback network means coupled between the output and the second input of the amplifier for feeding back output signals provided by the bias drive circuit to the amplifier and for modifying the output signal such that its waveform is a non-square wave that, when applied to the liquid crystal light valve, results in a square wave within the liquid crystal; and

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by way of an output, said output frequency signal (f_OUT) comprising said square wave input signal received by said amplifier first input; and

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector.

7. A bias drive circuit for use in an image projector comprising a liquid crystal light valve including a liquid crystal, said bias drive circuit comprising:

an amplifier having first and second inputs and an output, and wherein the first input of the amplifier is adapted to receive a square wave input signal;

a feedback network coupled between the output and the second input of the amplifier that is adapted to feed back output signals provided by the bias drive circuit to the amplifier and modify the output signal such that its waveform, when applied to the liquid crystal light valve, results in a square wave applied to the liquid crystal thereof; and

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by way of an output, said output frequency signal (f_OUT) comprising said square wave input signal received by said amplifier first input;

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector; and

wherein a divide number of the divide by N divider is an odd number and the input frequency signal is divided by two so that every other field is employed as a reference input to the bias drive circuit.

8. The apparatus of claim 7 wherein said input frequency signal comprises the vertical sync signal of the image projector.

9. A bias drive circuit for use in an image projector comprising a liquid crystal light valve including a liquid crystal, said bias drive circuit comprising:

an amplifier having first and second inputs and an output, and wherein the first input of the amplifier is adapted to receive a square wave input signal;

a feedback network coupled between the output and the second input of the amplifier that is adapted to feed back output signals provided by the bias drive circuit to the amplifier and modify the output signal such that its waveform, when applied to the liquid crystal light valve, results in a square wave applied to the liquid crystal thereof; and

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by way of an output, said output frequency signal (f_OUT) comprising said square wave input signal received by said amplifier first input;

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector; and

a variable gain circuit that is adapted to set the amplitude of the bias provided by the bias drive circuit to a cut-in operating point of the liquid crystal light valve.

10. Apparatus for driving a liquid crystal light valve including a liquid crystal, which liquid crystal light valve forms part of an image projector, said apparatus comprising:

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by way of an output;

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector; and

a bias drive circuit coupled to the frequency synthesizer that comprises;

an amplifier having first and second inputs and an output;

wherein the first input of the amplifier is adapted to receive a square wave input signal comprising said frequency synthesizer output frequency signal (f_OUT); and

a feedback network means coupled between the output and the second input of the amplifier, for feeding back output signals provided by the bias drive circuit to the amplifier and for modifying the output signal such that its output waveform is a non-square wave that, when applied to the liquid crystal, results in a square wave applied within.

11. The apparatus for driving a liquid crystal light valve including a liquid crystal, which liquid crystal light valve forms part of an image projector, said apparatus comprising:

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector;

a bias drive circuit coupled to the frequency synthesizer that comprises;

an amplifier having first and second inputs and an output;

wherein the first input of the amplifier is adapted to receive a square wave input signal comprising said frequency synthesizer output frequency signal (f_OUT); and

a feedback network coupled between the output and the second input of the amplifier, that is adapted to feed back output signals provided by the bias drive circuit to the amplifier and modify the output signal such that its waveform, when applied to the liquid crystal, results in a square wave applied thereto; and

wherein a divided number of the divide by N divider is an odd number and the input frequency signal is divided by two so that every other field is employed as a reference input to the bias drive circuit.

12. The apparatus of claim 11 wherein said input frequency signal comprises the vertical sync signal of the image projector.

13. Apparatus for driving a liquid crystal light valve including a liquid crystal, which liquid crystal light valve forms part of an image projector, said apparatus comprising:

a frequency synthesizer comprising;

a divide by two divider having an input adapted to receive an input frequency signal;

a phase detector having a first input coupled to an output of the divide by two divider;

a filter circuit having an input coupled to an output of the phase detector;

a voltage controlled oscillator having an input coupled to an output of the filter circuit;

a second divide by two divider having an input coupled to an output of the voltage controlled oscillator, said second divide by two divider providing an output frequency signal (f_OUT) from the frequency synthesizer by way of an output;

a divide by N divider coupled between the output of the second divide by two divider and a second input of the phase detector; and

a bias drive circuit coupled to the frequency synthesizer that comprises;

an amplifier having first and second inputs and an output;

wherein the first input of the amplifier is adapted to receive a square wave input signal comprising said frequency synthesizer output frequency signal (f_OUT); and

a feedback network coupled between the output and the second input of the amplifier, that is adapted to feed back output signals provided by the bias drive circuit to the amplifier and modify the output signal such that its waveform, when applied to the liquid crystal, results in a square wave applied thereto; and

further comprising a variable gain circuit that is adapted to set the amplitude of the bias provided by the bias drive circuit to a cut-in operating point of the liquid crystal light valve.